1. Field of the Invention
The present invention relates to a fine pattern exposure transfer technique required for the manufacture of semiconductor integrated circuits, liquid crystal displays, and the like and, more particularly, to an exposure method and a projection exposure apparatus, both using a projection optical system.
2. Related Background Art
Fine patterns such as semiconductor circuit patterns are formed by a method called photolithography. Especially, a reduction projection method is most widely used at the present time. In this method, an enlarged pattern formed on an original plate (reticle) is reduced in size by a projection optical system, and the reduced pattern is transferred onto an object to be exposed (e.g., a wafer). In this case, a photosensitive film (photoresist) is coated on the wafer surface to a thickness of about 1 .mu.m. The photoresist is sensitized in accordance with the contrast of a projection image of the reticle pattern. When the photoresist is developed, provided that the resist is of a positive type, portions (bright portions) of the resist, which are irradiated with light, are dissolved and removed, and portions (dark portions) of the resist, which are not irradiated with light, are left without being dissolved. In contrast to this, if the resist is of a negative type, bright portions are left, and dark portions are dissolved. In the current techniques, a positive photoresist is superior in resolution, stability, and the like to a negative photoresist. For this reason, in most cases, a positive photoresist is used in general wafer lithography.
The resolution (the minimum size of a pattern which can be transferred) in a conventional projection exposure method is represented by k.multidot..lambda./NA. An improvement in resolution has been achieved by decreasing .lambda. (exposure wavelength) and increasing an NA (the numerical aperture of a projection optical system). Recently, a phase-shifting method (e.g., Japanese Patent Publication No. 62-50811) and a multiple imaging amplification synthesizing method (i.e., the Super FLEX method disclosed at Spring Convention of the Japan Society of Applied Physics, lecture 29a-ZC-8, 9 (1991)) have been proposed, in which methods of improving the resolution without changing the numerical aperture NA and the wavelength .lambda. have been studied.
The phase-shifting method is an exposure method using a so-called phase shift reticle on which a desired pattern is formed by using a transparent portion which transmits light almost completely, and a phase shift transparent portion which shifts the phase of light transmitted through the transparent portion by about .pi. [rad] or an odd number multiple thereof. That is, in this method, an improvement in resolution is achieved by using a light amplification cancellation effect ((+1)+(-1)=0) between light transmitted through a normal transparent portion (phase=0, amplification=exp(i.times.0)=+1) and light transmitted through a phase shift transparent portion (phase=.pi., amplification=exp(i.pi.)=-1).
In the multiple imaging amplification synthesizing method, a light-absorbing member having a high transmittance near the optical axis and a low transmittance at the peripheral portion is arranged at a Fourier transform corresponding plane (to be referred to as a pupil plane hereinafter) in a projection optical system with respect to a reticle pattern. With this arrangement, the reticle pattern is exposed. The multiple imaging amplification synthesizing method is advantageous especially to a hole pattern (fine hole pattern) or an island pattern (fine remaining pattern).
Furthermore, a multiple imaging method has been proposed, in which one region on a wafer is sequentially exposed a plurality of times. In this method, the wafer is shifted little by little in the direction of the optical axis of a projection optical system in units of exposure operations, thus performing multiple imaging of even a defocused pattern image. The method allows an increase in focal tolerance (focal depth) especially when a hole pattern is to be formed in a positive photoresist.
When a conventional reticle pattern constituted by a complete light-shielding portion (chromium layer) and a complete transparent portion is to be exposed by using a projection optical system having no light-absorbing or light-shielding member at the pupil plane, the focal tolerance (focal depth) of a projection image is substantially determined by .+-..lambda./2NA.sup.2. Therefore, if the exposure wavelength .lambda. is decreased and the numerical aperture NA is increased in order to improve the resolution, the focal depth is inevitably decreased. However, there are projections having a height of about 1 .mu.m on an actual semiconductor integrated circuit surface (the surface of one irradiated region on a wafer). Furthermore, in consideration of the optical thickness (actual thickness divided by the refractive index .congruent.0.5 .mu.m) of a photosensitive material (photoresist) for pattern transfer, a focal depth of almost 1.5 .mu.m or more is required to perform accurate pattern transfer.
In a projection optical system most widely used currently, since the exposure wavelength .lambda. is set to be 0.365 .mu.m, and the numerical aperture NA is about 0.5, the focal depth is .+-.0.365/2.times.0.5.sup.2 +.+-.0.73 .mu.m. This value is equivalent to 1.46 .mu.m in width. It is apparent that the focal depth is insufficient in the existing condition. Therefore, the method of improving the resolution by decreasing the exposure wavelength .lambda. and increasing the numerical aperture NA is not practical because it sacrifices the focal depth.
Although the phase-shifting method allows an increase in focal depth as well as an improvement in resolution, it is difficult to apply the method especially to a hole pattern (a pattern for forming fine holes in part of a photoresist). This is because, provided that a high-performance positive resist such as the one described above is used, a transparent portion (hole pattern) is formed on a light-shielding portion (chromium layer) as an underlying layer constituting a reticle, and a phase shift transparent portion must be formed on a transparent portion surrounding the hole pattern. In addition, it is very difficult to manufacture a reticle pattern. This is because a total of two patterning operations are required, i.e., a patterning operation for forming light-shielding and transparent portions (patterning by etching a chromium layer), and a patterning operation for forming transparent and phase shift transparent portions (patterning by etching a shifter layer), and positioning is required between the two patterning operations.
These patterning operations are generally performed by an electron beam exposure unit (EB exposure unit). Pattern data to be processed by the EB exposure unit is constituted by drawing data for patterning of transparent and light-shielding portions and drawing data of patterning of transparent and phase shift transparent portions. That is, an enormous amount of data must be processed by the unit.
In contrast to this, if a negative resist is to be applied to the formation of a hole pattern, a phase shift reticle (e.g., a shifter light-shielding type) is easy to manufacture. More specifically, such a phase shift reticle can be manufactured by forming a hole pattern of a phase shift transparent portion on a transparent portion (bare glass surface portion) as an underlying layer. This manufacturing process requires only one patterning operation. However, as described above, since a negative photoresist is inferior to a positive photoresist in performance, and the above-described multiple imaging method cannot be used together, a satisfactory effect cannot be obtained upon exposure.
In the multiple imaging amplification synthesizing method using a projection optical system having a light-absorbing member formed at the pupil plane, both the resolution and the focal depth can be increased with respect to a hole pattern, and a positive photoresist can be used. However, the transmittance of a light-absorbing member at the pupil plane must be continuously and concentrically changed from the optical axis. Such a light-absorbing member is difficult to manufacture. In addition, the light-absorbing member generates or accumulates heat upon absorption of light. This heat is transferred to other members of the projection optical system to cause thermal deformation and a change in refractive index, resulting in a deterioration in imaging performance. Note that if a desired effect is to be obtained, the amount of light absorbed by the light-absorbing member arranged at the pupil plane reaches about 80% of the amount of light incident on the projection optical system.
Furthermore, a so-called halftone type phase shift reticle has been proposed as a phase shift reticle suitable especially for exposure/transfer of a hole pattern (Spring Convention of the Japan Society of Applied Physics lecture 29p-ZC-3 (199)). A halftone type phase shift reticle is a reticle having an underlying layer (halftone) whose phase is shifted by .pi. and which has a light absorption property (or reflection property) of a predetermined amount, and a fine transparent pattern (whose phase is not shifted) formed with respect to the underlying layer. Therefore, a fine hole pattern (a bright pattern for a positive resist) is formed by interference between the respective beams of light transmitted through the fine transparent portion and the remaining halftone portion. With this reticle, the resolution and focal depth of the hole pattern in exposure and transfer can be increased. Although the halftone type phase shift reticle is completed by one patterning operation and can be easily manufactured, light transmitted through the halftone portion leaks to a dark portion (non-pattern portion), resulting in a deterioration in the contrast of a projection image. In addition, the above-described multiple imaging method is difficult to use with this reticle because of this leakage light to the dark portion.
The above description is associated especially with the formation of a hole pattern. This is because a hole pattern among various types of patterns treated in a wafer lithographic process is most difficult to form. Therefore, if a hole pattern can be reduced in size, the overall integrated circuit can be easily reduced in size accordingly.